Famous Durango and Silverton narrow guage steam railroad. Ride from Durango through the mountains of Colorado to Silverton, Co.
 Durango & Silverton

Orange Empire is a large museum in Perris, CA. Many exhibits and operating train rides. 
Orange Empire Railway Museum

Western America Railroad Museum in Barstow, CA. Indoor and outdoor exhibits. No train ride.
  Western America Railroad Museum

Cumbres & Toltec Scenic Railroad. Narrow gauge steam railroad from Chama, NM to Antonito, CO
Cumbres & Toltec

Heber Valley Railroad, near Salt Lake City offers train rides through the Heber Valley.
Herber Valley Railroad

Nevada State Railroad Museum in Carson City, NV
NSRM Carson City

Nevada Northern Railway – The Ghost Train of Old Ely
Nevada Northern Railway

Moving the New Nation by Rail – Article

Western Pacific RR Museum in Portola, CA.